1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. More specifically, the invention relates to a method of measuring a pattern shift that may occur during the manufacturing process of semiconductor devices.
2. Background of the Related Art
In the manufacturing process of semiconductor devices, especially those involving formation of an epitaxial layer, a buried layer may be formed in order to isolate a device and increase the threshold voltage. The buried layer is generally formed so as to be precisely aligned with layers to be formed on top of the buried layer.
FIGS. 1a to 1c are cross-sectional views explaining a conventional method of forming a buried layer, an epitaxial layer, and regions stacked on top thereof.
As shown in FIG. 1a, an initial oxide layer (not shown) is formed on a P-type semiconductor substrate 10. After a mask for formation of a buried layer is formed on a desired area of the initial oxide layer, the initial oxide layer is patterned using the mask. Using the patterned initial oxide layer as a mask, N-type ion injection and drive-in diffusion of impurities are performed to form an N-type buried layer 12.
Thereafter, as shown in FIG. 1b, the patterned initial oxide layer is removed and a P-type epitaxial layer 14 is formed on the resulting formation of the N-type buried layer 12.
Then, as shown in FIG. 1c, a well region 16a is formed on a desired area of the P-type epitaxial layer 14.
On the other hand, as the N-type buried layer 12 is formed, a stepped portion A occurs between the buried layer surface and the substrate surface. In the epitaxial process for forming the epitaxial layer, the growth direction of single-crystalline silicon may not be perpendicular to the substrate, depending upon orientation of the substrate and process conditions. In this case, the above stepped portion (‘A’in FIG. 1a) may shift to another place (the distance of the shift is designated ‘B’in FIG. 1b) from its original position.
Therefore, since a location of the stepped portion shifts between the buried layer surface and the substrate surface, a layer to be formed within the epitaxial layer 14 (i.e., a well region or a plug implant) may be formed at a position 16a in FIG. 1c, which has been shifted from a position 16b in FIG. 1c (i.e., from the correct position in the process).
Thus, in order that the well-region, which is to be formed adjacent to the buried layer 12, is formed in the correct position, an offset must be employed to compensate for the shifted distance. At this time, the above offset is determined enduring wafer loss when in the process setup, and must be continuously monitored because it may change in various ways, depending upon the process conditions.
In case of mass production, however, every lot may be subject to wafer loss in order to continuously monitor the offset, thereby resulting in additional costs and increased time consumption.
Therefore, a fixed offset, established during the process setup phase, is applied in the course of the process. Because of this, alignment between the buried and epitaxial layer and layers formed on top thereof may be less precisely made.